scale at which we’re interested in imaging. The measurement has to be made and corrected fairly quickly to overcome the movements.’
20/20 vision At the Visual Optics and Biophotonics laboratory at the Spanish Council for Scientifi c Research (CSIC) in Madrid, researchers are using ray tracing and Shack-Hartmann aberrometry to gain a better understanding of mechanisms of the visual system, such as the limits to spatial vision. Ray tracing is an alternative wavefront analysis
method to Shack-Hartmann. It is a sequential technique in which rays of light are shone through the pupil and the level of displacement is estimated with respect to a central ray at the plane of the retina. This is ingoing aberrometry, in the sense that aberrations are measured as the wavefront travels into the eye. The laboratory is using ray tracing to understand how aberrations in the eye alter with factors like refractive error, accommodation or ageing.
Ray tracing has the advantage of having a
high dynamic range, allowing large wavefront aberrations to be measured. There is also no crossing over of light points to other areas in the system, as can occur with Shack-Hartmann, because every spot is separated in time. However, it is slower than a Shack-Hartmann system, which provides all the information in one snapshot. ‘We’re using the high dynamic range of ray
tracing for clinical cases like cornea pathologies or highly aberrated eyes,’ says Professor Susana Marcos, head of the laboratory. ‘Having the real- time capability of the Shack-Hartmann system is very valuable for other applications involving dynamic measurements and accommodation or real-time correction of aberrations.’ The laboratory uses a Haso Shack-Hartmann
sensor from Imagine Eyes for studies measuring dynamically ocular aberrations as a function of the accommodative demand, i.e. stimulating accommodation through forcing the subject to focus dynamically on near and far objects and looking at how the aberrations in the eye change, and what the accommodative response is. Imagine Eyes, based in Orsay, France, provides
aberrometers and adaptive optics for ophthalmic applications. The Haso system used in the lab is capable of running at 50Hz, although in the laboratory’s experimental setup, the sensor runs at 15Hz. ‘Several other studies in our laboratory deal with how the eye’s ability to resolve objects is affected by the correction or presence of aberrations,’ says Marcos. ‘In this system, we’re using an adaptive optics deformable mirror in combination with Imagine Eyes’ Haso Shack-
Microscopy Characterisation Beam alignment
the adaptive optics system at the visual optics and Biophotonics laboratory, spanish council for scientifi c research (csic) is being used to gain a better understanding of basic mechanisms of the visual system
Hartmann sensor. We also have a psychophysical channel – a monitor that projects stimuli that the subject sees throughout the setup. We correct the aberrations of the subject with the adaptive optics mirror and evaluate any improvements in visual acuity (at different contrasts and luminances) or visual performance, including recognition of faces and facial expressions, as well as how much the subject’s impression of sharpness is modifi ed.’ The research has implications for refractive
corrections and for treatment of different conditions of the eye. ‘An aim of refractive surgery or future generations of intraocular or contact lenses is the correction of low order aberrations, such as defocus or astigmatism, but also other higher aberrations of the eye,’ says Marcos. ‘With this equipment, we can simulate a perfect refractive procedure and measure what the implications would be for the subject’s vision.’ ‘The hypothesis is that by correcting higher
aberrations of the eye – those aberrations measured with wavefront sensing – you could improve vision,’ continues Marcos. ‘But this has to be tested and this is one of the aims of these experiments.’
adaptive optics Researchers at 4D Optics and the laboratory at CSIC are using wavefront sensing in combination with adaptive optics – the sensor detects any aberrations in the incident light and the optics correct for those distortions to gain a better image. John Taranto of Thorlabs says: ‘The benefi t of
using a wavefront sensor as a feedback mechanism is that, in theory, only one measurement is required, from which the voltage map can be determined and used to make the correction in the mirror. Therefore, potentially, a correction can be made every time the sensor makes a measurement.’ Thorlabs provides measurement and control solutions for the photonics industry, including wavefront sensors. The company’s AO kit is based on a Shack-Hartmann wavefront sensor and ➤
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For high-precision wavefront analysis & adaptive optics, only Imagine Optic provides a complete line of products and solutions to meet all of your needs.
Our HASO™ wavefront sensors combine unmatched accuracy, wide dynamic range and ease-of- use for wide-ranging applications. Combined with our selection of deformable mirrors and software packages as part of your adaptive-optics loop, they enable you to optimize the performance of optical systems from microscopy to high-power lasers and beyond.
Visit us on stand C1-121 at LASER World of Photonics. For more information or to find the sales representative nearest you, visit:
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